The present invention relates to a method of manufacturing an integral asymmetrical membrane for the separation of liquid or gaseous mixtures, and includes the steps of mixing a membrane-forming agent with a solvent, introducing or accommodating the membrane-forming agent/solvent mixture in a geometrical form, and then bringing the form into contact with a precipitating agent and providing posttreatment.
A manufacturing process of this type is disclosed in German Pat. No. 34 20 373. The membranes manufactured pursuant to this process can be used only for the separation of gases. Furthermore, these membranes have a thick, nonporous skin, so that due to the thereby resulting low rates of permeation, these membranes are not suitable for, for example, the production of composite membranes. On the other hand, the manufacture of hollow-filament membranes can be carried out only with great difficulty and is very inconvenient, since the viscosities of polymer solutions that can be achieved are not adequate, and must be precipitated in organic media (for example to protect against explosion).
The manufacture of hollow-filament, composite, or flat membranes using various polymers, such as cellulose acetate, polysulfone, and polyamide resins can be carried out with more or less success (Egli, S. et al, "Gastrennung mittels Membranen", reprint from SWISS CHEM. 6 (1984) No. 9, 89-126). However, the use of these polymers is poorly suited for the manufacture of composite membranes due to the fact that these polymers have an unsatisfactory chemical stability, and often produce membranes with too low of a permeability. Furthermore, with this known method it is basically difficult to produce hollow-filament membranes having nearly the same permeability as comparable flat membranes, since hollow-filament membranes are self supporting and therefore generally must be thicker and must have a denser morphological structure. In addition, the resistance of these hollow-filament membranes to pressure is not very great. In some cases, the high viscous solution required for the manufacture of hollow-filament membranes can be achieved only by the addition of a further polymer to form a spinning solution. Difficulties are involved with this addition, since normally different polymers cannot be mixed together in a solution (for example polysulfone with polyvinylpyrrolidone, or PVP); in addition, the added polymer must again be removed after the precipitation.
It is therefore an object of the present invention to improve, for example, the method in such a way that it can be used to manufacture membranes having any desired geometrical shape, an adjustable morphologic structure, a thick or microporous skin that is formed on the outside and/or on the inside, high permeability, and, for hollow filaments, a good mechanical strength.